So there is no need to send any blank frame?
If this was the only thing sending a blank frame, then all blank frame code should be removed entirety.

The only other place we send blank frame is
https://github.com/obs-ndi/obs-ndi/blob/develop/src/obs-ndi-source.cpp#L940

void ndi_source_thread_stop(ndi_source_t *s)
{
	if (s->running) {
		s->running = false;
		pthread_join(s->av_thread, NULL);
		if (!s->config.remember_last_frame) {
			obs_source_output_video(s->obs_source,
						s->config.blank_frame);
		}
	}
}

Is this also not necessary?
I think the intention here is to intentionally render an empty frame when the source is switched away and "remember last frame" is not selected.
ie: to blank the frame.
But why would an audio only source need to blank a video frame it never had?
The only reason I can see to do this is if the config is changing from a normal a/v source to an audio only source.

FFR: PROP_BW_AUDIO_ONLY was first added 2017/11/29 in:
bafaf47#diff-6a31bbe65b845c97228452424d004830270b76230165c75ec3d1c67b2c6420e6R304

Then blank_video_frame() was first added 2018/03/24 in:
7d1cf13#diff-6a31bbe65b845c97228452424d004830270b76230165c75ec3d1c67b2c6420e6R446

The title of that 2018/03/24 PR is

source: reorder functions + clear last video frame in Audio Only mode

There has got to be a reason why this was added.
Thinking...

Maybe again only if the config is changing from a normal a/v source to an audio only source... something that would happen often while dev/testing this, and one time for most users changing from the default.

@Palakis @tt2468 ?